Filters have long been used industrially, particularly in liquid process industries, such as pulp and paper manufacturing operations, to remove solids from a process liquid by passing the liquid through a filter element through which solids above a preselected size cannot pass. Typically, the filter element is contained in a closed housing to which the process liquid to be filtered is supplied under pressure. It is well known that contaminants and other particulate matter removed from the liquid to be filtered will eventually build up on the filter surface. Such build up of solids, if unchecked, normally continues until liquid flow through the filter is insufficient or is substantially blocked. Thus, the filter element requires periodic cleaning to remove solids.
To effect such cleaning, it has been known to stop filtration, remove the filter element from the housing, hand or machine clean same, and then reassemble the apparatus prior to resuming the filtering operation. However, this procedure is costly in labor and filter unit downtime, and is virtually useless for liquids containing a high concentration of solids which quickly degrade or block filtration.
Several attempts have been made in the past to provide more practical filter cleaning measures.
In one, processed liquid flow through the filter is periodically stopped and for a brief time backwash liquid is passed in a reverse direction through the filter element to dislodge solids accumulated thereon. However, uniform application of backwashing liquid to the surface of the filter element has been a difficulty in the past. Also, this method requires periodic shutdown of the filter unit.
Such need for periodic shutdown of the filter has been overcome in the past by using an array of several filter units in parallel, with only one at a time shut down backwashing, such that the array is in partial operation at all times. However, several smaller filter units may be more costly than one large one and tend to require more space and installation effort.
Past attempts to equip single filter units for continuous filtration and self-cleaning have included applying a liquid to be filtered to the outside of a rotating cylindrical filter element and relying on centrifugal force to thaw particles outward off the drum. Attempts have been made to increase the cleaning effectiveness of this rotating filter element by addition of ancillary cleaning structures.
For example, Walters U.S. Pat. No. 3,353,771, directs incoming liquid in a small cross-section high velocity jet at the outside of the filter element to peel caked solids off the outside of the rotating filter element, has a special housing cross section, and has multiple liquid deflecting elements within and rotating with the filter element in an attempt to assist removal of solids cake from the outside of the rotating filter element.
As a further example, Jarman U.S. Pat. No. 3,847,817 directs incoming liquid to be filtered with special baffles to again attempt to peel solids cake off a rotating filter element, has a special fixed liquid wedging blade within the filter element to force filtrate reversely out through the filter element, and has an axially extending and angularly directed scraper member to peel solids off the filter element and into an axially extending solids receiver.
As a further example, Barry U.S. Pat. No. 3,338,416 horizontally rotates a filter element, introduces liquid to be filtered to the rotating filter element substantially along the entire axial length thereof by suitable baffling, has a fixed axial array of backwash jets within the filter element to direct backwash liquid radially outward through the filter element and a fixed axially extending slot in a closely adjacent partition in an attempt to knock solids off the rotating filter element into the slot.
Each of the last-mentioned three patents require relatively complex and expensive structure to rotatably mount and drive the cylindrical filter element and to support, either rotatably or fixedly, the described ancillary elements inside and outside of the rotating filter element. Moreover, care must be taken to avoid vibration and fatigue due to imbalance of the rotating filter element by uneven distribution of solids cake thereon.
Walters U.S. Pat. No. 3,491,889 attempts to achieve self-cleaning of the filter element without rotation of the filter element by rotating axially extending scooper plates near the interior surface of the filter element, the plates being angled from the filter element surface in an attempt to wedge filtrate radially outward through the filter element wall and against solids on the outer surface thereof. In testing of apparatus of this type, the present Assignee has encountered difficulty in maintaining continuous filtering. It was found that excessive horsepower and rotating speed was needed to generate sufficient energy to dislodge even the least tenacious types of particles from the filter element wall and the tested apparatus was found ineffective for dislodging sticky or fibrous particles from the filter element wall.
Accordingly, the objects of this invention include provision of:
1. A self-cleaning filter unit particularly adapted for filtering of liquids containing solids in relatively high concentration and/or containing solids particularly prone to sticking to or entangling with the filter element mesh in a manner to quickly load or blind the filter element.
2. An apparatus, as aforesaid, in which helical downward swirling of solids bearing liquid outside the filter element, assisted by gravity, increases the concentration of solids in a limited zone of the filter unit for continuous removal during filtering, but wherein if desired intermittent removal of concentrated solids can instead by selected.
3. An apparatus, as aforesaid, in which excessive build-up of solids material on the input side of the filter element is sensed and automatically causes an increased flow from the solids collection zone out of the filter element.
4. An apparatus, as aforesaid, of a relatively simple structure in which only the central element is rotatable and in which the filter element is fixed and does not require rotatable support structure.
Other objects and purposes of the invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawings.
The objects and purposes of the invention are met by providing a self-cleaning filter assembly for removing solids from an incoming liquid to be filtered wherein a filter element is fixed substantially coaxially within a housing and spaced from the peripheral wall thereof by an annular solids collection space in turn communicating with a solids outlet. An inlet introduces pressurized liquid to be filtered circumferentially into the solids collection space. A shower assembly rotates coaxially within the filter element and includes an eccentric portion having jets distributed along the length of the filter element and aimed outward close to the inner surface of the filter element. Backwash liquid enters the rotating shower assembly under pressure for continuously pushing solid off the outer surface of the filter element as the shower assembly circumferentially advances in the housing. Filtrate from inside the filter element passes through a filtrate outlet extending from the housing. A solids outlet valve responds to a preselected increase in liquid pressure differential between the inlet and filtrate outlet by opening to increase venting of the annular solids collection space. A filtrate outlet valve may simultaneously close to direct all liquid flow into the housing out the solids outlet.